The crystal structure of Transforming Growth Factor-beta (TGF-beta 2) has been determined at 2.1[unreadable] resolution. The molecule is a homodimer, with each subunit having an unusual structure of a kind not previously observed in other proteins. Comparison with other members of the TGF-beta family and with the members of the superfamily such as activins and inhibins indicate that they probably adopt very similar structures. A preliminary identification of the receptor binding site has been made. The crystal structure of the complex of the FAB HyHEL-5 with chicken lysozyme has been refined. Also, the structure of the complex of this antibody with a mutant lysozyme has been determined. In the mutant an arginine present in the wild type has been replaced by a lysine (R68K). A comparison of the mutant and the wild type has been made in order to explain the rather large effect of this conservative mutation on the affinity.